1. Field of the Invention
The present invention relates to a method for shielding a high-power laser apparatus. It also relates to a shielding device using this method, as well as a high-power-laser optical system employing such a device.
The field of the invention is that of high-power lasers, and more particularly of ultra-intense lasers.
2. Description of the Related Art
Apparatuses based on ultra-intense lasers are subject to risks of damage. These risks are of two types:                During amplification of the pulse: when the distribution of pulse energy incident on an amplifier is non-uniform, and contains “hot spots”, interaction with the crystal creates a very high risk of damage. Such damage leads to the stopping of the laser and the need to replace the damaged optics: this therefore causes delays, as well as costs that can amount to several tens of thousands of Euros when the damaged elements are the amplifier crystals or the compression gratings.        Beam return from the experiments: it is by no means rare that a part of the light reflected or generated during the interaction of the laser beam with the experimental target returns into the laser and is itself amplified. This phenomenon, uncontrolled and dependent on the type of target and its geometry, leads, when it occurs, to a high risk of damage of the optical elements of the laser circuit.        
The solution currently used for combating the risks of damage is the use of a spatial filtering positioned in a focusing plane located in front of each of the amplifiers or sensitive optical elements to be shielded [1]. The smaller the spatial filter (diaphragm of any type) relative to the diffraction of the beam, the more effective its use is for reducing the risks of damage.
Conversely, the smaller the size of the spatial filter, the higher the energy loss of the incident beam, especially when the latter possesses optical aberrations and is not limited by diffraction. Moreover, when the power of the beam increases as it passes through the amplifiers, the beam fluence at the point of focusing (in the plane of the filter pinhole) becomes stronger and stronger, and the part of the energy that is blocked by the filter pinhole (since the beam is not limited by diffraction owing to the aberrations) can lead to the formation of a plasma and destruction of the pinhole (plasma then fusion and closing-up of the pinhole).
Thus, in fact laser specialists use, on high-power lasers, filter pinholes of large diameter compared with the diffraction limit, for two reasons: to avoid losing too much flux and to shield the pinhole from destruction at each firing. Therefore neither homogenization of the energy distribution at the entrance of the amplifiers, nor shielding against beam returns from the target is optimal. Thus, the filter pinhole technique currently used is not satisfactory for dealing with the problems of shielding the optical elements of lasers.
The filter pinholes used commonly are flat metal disks with a calibrated pinhole at the centre, mounted on a precision movement in the three axes so that they can be aligned on the optical axis of the laser beam. This solution has the drawback that it creates a plasma on the edges of the disk, which eventually destroys it, if the energy deposited reaches the damage threshold. Furthermore, the damage threshold of the disk largely depends on the optical quality of the surface. In ultra-intense and ultra-short systems, starting from a repetition frequency of a few Hz, and starting from input energies of a few hundred millijoules, the filter pinhole is damaged irreversibly. The solution proposed for filtering at high fluence is to use pinholes, metallic or dielectric, of conical shape, to increase the area of interaction between the beams to be rejected and the spatial filter. The material of which the filter pinhole is composed implies various filtering schemes and behaviours from the standpoint of flux resistance.
Conical filter pinholes made of metal [1, 2] are used in laser circuits with very low repetition frequency of the NIF and LMJ type. Filter pinholes of conical shape were validated in more than 500 firings on the pilot installation of the LMJ, the Laser Integration Line (LIL). Elimination of the high spatial frequencies is based on the principle of deflection by the subdense plasma created by the interaction of the beam with the walls of the cone. Modelling of the pinhole and of the effect of the energy deposited, contained in the high spatial frequencies, is necessary because plasma expansion determines the dynamics of the reflecting surface. This approach makes the engineering of the filter pinhole very simple because delegating the role of reflector to the plasma makes it possible to relax the constraints on the quality of the reflecting walls of the pinhole. This configuration has never been validated at high repetition frequency. The energy conditions of operation of these pinholes must be very well defined and stable. The fluence incident on the walls of the pinhole must be sufficient to create a plasma, as a fluence that is too low would merely deposit the energy and would ultimately destroy the pinhole. Conversely, the plasma must not be too dense, so that the expansion time is not too rapid and the pinhole is closed up, and a part of the beam is cut, as by the effect of a high-speed shutter.
In systems at 10 Hz, the number of reproducible firings required to validate the proper functioning of the pinhole increases relative to the single-shot systems by several orders of magnitude, and for this reason other solutions are proposed, which do not involve ablation of material, however slight.
Dielectric filter pinholes [3, 4] are a solution under consideration which reduce the deposition of energy in the material, and which is as a consequence less limited by the resistance of the material used to the flux. The effective field and the transmission in the case of reflection on a dielectric depend on the polarization and on the angle between the incident beam and the normal to the surface, according to Fresnel's formulae. Under these conditions the use of a conical pinhole with grazing incidence makes it possible to clean, in a spatial filter under vacuum, much higher energy pulses than with the standard solution. The microscopic quality of the surface of the reflector determines the resistance to flux and the
performance of the pinhole, because of local increases in the electric field due to defects.